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Impact of Intermetallic Growth on the Mechanical Strength of
Pb-Free BGA Assemblies
Patrick Roubaud, Grace Ng, Greg Henshall Hewlett Packard Ronald
Bulwith, Robert Herber - Alpha Metals Swaminath Prasad, Flynn
Carson ChipPAC Sundar Kamath, Alexander Garcia Sanmina
Presented at APEX 2001 on January 16-18, 2001 in San Diego,
CA
ABSTRACT
The increasing industry awareness of lead-free activities has
prompted users and suppliers to investigate lead-free solder
systems in detail. The need to understand the intermetallic
formation, structure, and its impact on the reliability of solder
joints was the driving force behind this study. If intermetallics
grow to sufficient thickness, fracture can occur during handling,
shipping or service. This problem has been studied in detail for
the tin-lead solder system, but it is not well understood for
candidate lead-free solders. Test boards populated with SMT
lead-free components have been aged at 125 C and 150 C for
intervals of time up to 32 days. The printed circuit boards had OSP
surface finish. Four ball metallurgies (Sn-Ag, Sn-Cu, Sn-Ag-Cu and
Sn-Ag-Cu-Bi) have been evaluated for the Plastic Ball Grid Array
(PBGA) packages. The solder paste used in the SMT process was the
Sn-Ag-Cu. The thickness of the intermetallics has been measured for
each time interval and the activation energy for their growth has
been computed. Additionally, these interconnections have been
screened for catastrophic failure using 4-point bend tests, and
compared with parts soldered with conventional Sn-Pb solder. The
implications of these results for the reliability of lead-free
interconnections are discussed. Introduction Eutectic Tin-Lead
solder became a de-facto standard in the electronic industry
because of its unique combination of material properties and low
cost. In recent years, environmental concerns have been raised
regarding the use of lead-containing solder in electronic products.
The European community has a proposed Waste in Electronic and
Electric Equipment (WEEE) Directive that restricts the intentional
use of lead in electronic products after January 1, 2008. The
European Parliament has not yet approved the WEEE Directive.
Nonetheless, movement away from lead-bearing solder is advancing,
driven
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mainly by competitive pressures in consumer electronics and
concerns about the lead in discarded electronic products. In Japan,
the Japan Electronic Industry Development Association (JEIDA)
published a road map to achieve lead replacement by 2005 [1]. In
this effort toward lead elimination, a variety of Pb-free printed
circuit board (PCB) finishes, package lead surface finishes, and
Pb-free Ball Grid Array (BGA) metallurgies have been developed. It
is not clear how the properties of the solder joints will be
altered when the Pb-free solders interact with these newly
developed metallurgical surfaces. For example, the effect of
alloying elements on the aging behavior is not well understood.
Furthermore, the temperatures used during most lead-free soldering
processes are higher than those used in corresponding Sn-Pb
process, which may lead to thick intermetallic layers. Those
intermetallics are brittle and may compromise the joints mechanical
integrity, leading to failure at unacceptably low mechanical
stresses, such as those potentially applied during shipping,
handling, or mild mechanical shock. Since the trend in modern
electronics is miniaturizing solder joints, the role of these
compounds may become more important as the thickness of the bulk
solder is reduced. Therefore, it is important to determine if
lead-free solder joints are subject to intermetallic growth
kinetics significantly faster than that of Sn-Pb and whether these
compounds lead to fragile joints. Intermetallic Growth Kinetics
Experimental Procedure 388 I/O PBGAs packages, specially
developed for the lead-free process [2], were used for this study.
These packages were ball attached with either the standard
63Sn-37Pb alloy or one of four lead-free alloys selected for this
study. The PBGAs were then attached to conventional PCBs using the
standard 63Sn-37Pb (peak temperature during assembly: 220 C) and a
lead-free Sn-4Ag-0.5Cu (peak temperature during assembly: 250 C)
solder paste. The boards had a lead-free Organic Solder
Preservative (OSP) finish and the pads were non-soldermask defined
(NSMD). More details about the package, the boards and the
assembling process can be found in [3]. The thermal aging of the
samples was performed in air at temperatures of 125 C (+/- 1.1 C)
and 150 C (+/- 1.1 C). For this aging study, 84 samples divided
into the 6 different combinations listed in Table 1 were built.
After 1, 2, 4, 8, 16 and 32 days, samples of each type were removed
for metallographic examination. A sample of each type was also
examined without aging (aging time = 0 day) to serve as a
baseline.
BGA Solder Ball Metallurgies
Surface Mount Solder Paste Peak temperature during the Assembly
Process (C)
63Sn-37Pb 63Sn-37Pb 220 63Sn-37Pb
Sn-4.0Ag-0.5Cu Sn-2.5Ag-1.0Bi-0.5 Cu
Sn-0.75 Cu Sn-3.5Ag
Sn-4.0Ag-0.5Cu
250
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Table 1 Samples for the activation energy and the 4-point
bending tests. The BGAs were removed from the motherboard using a
handheld 'Dremel' cutting tool. A low speed diamond saw was used to
cut through the component and expose the solder connections for
metallographic preparation. The component segments were then potted
in a low exotherm metallographic potting compound as to minimize
the introduction of thermal artifacts on the solder microstructure.
Once the sample was potted and marked with the appropriate
identification, it was roughly ground to the proximity of the
solder connections to be examined with an 80 grit abrasive belt.
Further wet grinding on successively finer grit abrasive papers was
carried out from 240 grit through 600 grit abrasives. Following the
grinding, the samples were successively polished with 3 micron and
then 1/4 micron diamond abrasives on a rotating polishing wheel
covered with a synthetic polishing cloth. The final polishing step
was then carried out on an automated vibratory polishing table
using a 0.05-micron Alumina abrasive. The samples were then etched
to more clearly reveal the reaction interfaces with the solder for
the microscopic examination and the measurements of intermetallic
layers thickness. Microscopic examination was carried out using a
Zeiss Ultraphot II metallurgical microscope fitted with Optronics
digital cameras and image capture software.
Results and Discussion Figure 1 shows a general view of a
lead-free solder joint. The Cu6Sn5 intermetallic sublayer is
clearly visible on each sample. The Cu3Sn sublayer is noticeable
only for the sample annealed at 150 C. Figure 2 contains a
micrograph of a lead-free ball aged at 150 C for 32 days.
Comparison of the 63Sn-37Pb joints and the lead-free joints shows
that the initial thickness of the intermetallic layer is not
significantly impacted by the higher temperature used during the
lead-free assembly process. All the values are in a range of 1.6 to
2.3 microns, as shown in Figure 3. The growth of these
intermetallic layers can be modeled using parabolic growth kinetics
[4]:
tDww += 0 (1) Where: w = thickness of the intermetallic
layer
w0 = initial thickness of the layer D = Diffusion coefficient t
= time
Figure 4 shows the intermetallic layer growth as a function of
the square root of the annealing time. The straight lines represent
a linear regression plot of the total intermetallic thickness (i.e.
the combined thickness of the Cu6Sn5 and Cu3Sn sublayers). These
data indicate that the intermetallic layer growth is comparable
from one sample to another. There are no significant differences in
growth rate due to the paste metallurgy, the ball metallurgy or the
peak temperature during the reflow process. Although these
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data are limited, it does not appear that intermetallic layer
growth is faster for the lead-free joints than for the control
Sn-Pb joints, and may in fact be somewhat slower. For each joint
type, D was measured from the slopes in figure 4 at 32 days. The
activation energy Q was measured assuming Arrhenius behavior, by
plotting ln(D) versus 1/T. The results are reported in Table 2.
Surface Mount Solder Paste
BGA Solder Ball Metallurgies
Activation Energy (kJ/mol)
63Sn-37Pb 63Sn-37Pb 45 63Sn-37Pb 48
Sn-4.0Ag-0.5Cu 33 Sn-2.5Ag-1.0Bi-0.5 Cu 68
Sn-0.75 Cu 50
Sn-4.0Ag-0.5Cu
Sn-3.5Ag 31 Table 2 Measured Activation energies for the solder
joint type investigated.
The value obtained for the Sn-Pb system (45 kJ/mol) was found to
be in accordance with the literature [5]. Qualitatively, it appears
that the activation energies for the lead-free solders are within
approximately +/- 50% of the value for 63Sn-37Pb. It also appears
that the joints containing the highest amount of Ag (Sn-3.5Ag and
Sn-4.0Ag-0.5Cu ) have the lowest activation energy. However, the
scope of this study does not allow us to draw definite quantitative
conclusions. The small number of test points and the design of the
samples prevent us from doing so. In addition, we did not separate
the contributions from the Cu3Sn and Cu6Sn5 sublayers. Mechanical
Strength As a preliminary investigation of the mechanical strength
of Pb-free interconnects, four-point bending tests were performed
to simulate the shipping and handling environment. Note that these
tests were not meant to investigate the thermal fatigue or creep
properties of the solder joints. Such investigations are underway
and will be reported elsewhere.
Experimental Procedure The test vehicle consisted of a 35mm x
35mm, daisy-chained 388 I/O PBGA package mounted on a 203 x 72 mm
PCB. The four lead-free metallurgies described previously were used
for the balls of the BGAs, and the Sn-4Ag-0.5Cu lead-free solder
paste was used to attach the parts on the PCB. For control, BGAs
with traditional 63Sn-37Pb solder balls were also assembled, with
63Sn-37Pb solder paste and with the Sn-4Ag-0.5Cu lead-free solder
paste. In all cases OSP was used as the board finish. The
assemblies were tested in 4-point bending on a screw-driven Instron
testing machine (model # 5566). The crosshead speed was set to
0.762 mm/min (0.03 in/min). Figure 5
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illustrates the experimental setup. A total of 4 samples per
joint metallurgy (see Table 1) were tested. A multi-meter was used
to monitor the continuity of electrical signals during the
experiment. The BGA assemblies were tested to failure either
discontinuity of electrical signal or separation of the BGA from
the PCB. The experiments were run at room temperature.
Results and Discussion A representative load vs. displacement
plot for the 4-point bend test is shown in Figure 6. As a test
assembly was loaded, the load vs. displacement plot followed a
relatively straight line as both the PCB and BGA were bent. The
load ultimately reached a maximum value, where the BGA separated
from the board, and then decreased. The maximum load is defined as
the fracture load. Table 3 lists the average fracture load for the
Pb-free solder joints and the Sn-Pb control. Based on the results,
the alloys can be broken into two groups. Joints containing
Sn-2.5Ag-1.0Bi-0.5Cu and Sn-3.5Ag alloys exhibited fracture loads
in the range of 600N to 700N, similar to those with the eutectic
63Sn-37Pb. The second group, consisting of Sn-Ag-Cu and Sn-Cu,
exhibited fracture loads ranging from 900N to 1050N. Standard
deviations for the average fracture load of each alloy ranged from
27N-190N.
Paste BGA Sample # Average Fracture Load (N) Standard Deviation
Sn-Pb 63Sn-37Pb 1 2 3 4 691.6 93.3
63Sn-37Pb 5 6 7 8 655.6 102.5 Sn-4.0Ag-0.5Cu 9 10 11 12 935.0
190.4
Sn-2.5Ag-1.0Bi-0.5 Cu 13 14 15 16 682.5 27.3 Sn-0.75 Cu 17 18 19
20 1046.9 125.6
Sn-Ag-Cu
Sn-3.5Ag 21 22 23 24 716.4 126.3 Table 3 Average and standard
deviation of fracture load for Sn-Pb control and Pb-free Alloys
All of the samples exhibited the same failure mode. The copper
pads, along with some adjacent epoxy, were pulled out of the PCBs.
Figure 7 provides a schematic drawing of this pad-pull-out failure
mode. Figure 8(a) shows the fracture surface on the PCB side where
the copper pads were pulled out, revealing the fiberglass
underneath. Figure 8(b) is the mirror image on the BGA side. The
copper traces and pads were pulled out of the PCB during the
mechanical flexure test. Epoxy residue on the copper traces/pads
showed imprints of the fiberglass that was underneath. In some rare
cases, the corner solder joint exhibited an interfacial fracture or
a pad-pull-out fracture on the BGA package side. Interfacial
fracture of a corner joint was observed for the Sn-0.75Cu and
Sn-3.5Ag alloys. Figures 9 illustrates this failure mode. The
pad-pull-out failure mode observed for the vast majority of samples
indicates that the Pb-free solder joints are mechanically sound.
Aside from the few cases of interfacial fractures along the corner
solder joints, the solder joints were stronger than the epoxy
holding the Cu pads to the PCB. Even in cases of interfacial
fracture, the measured
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fracture load was as high as that exhibited by the eutectic
Sn-Pb control. Therefore, the interface between the solder and the
BGA or PCB can withstand a substantial amount of stress. Since all
but a few samples exhibited the pad-pull-out failure mode, one
would expect similar fracture loads across the samples. However,
the results showed two distinct groups of fracture load. Due to the
small number of test samples and the large standard deviation
within each group of alloy, a conclusive quantitative comparison of
the mechanical strength of Pb-free solders and eutectic Sn-Pb
solder cannot be drawn. Conclusions For the combinations of solder
joint metallurgies studied, the following conclusions can be drawn:
1. The high temperature (250 C) reached during the lead-free
assembly process does not lead to a significantly higher thickness
of the intermetallic layer at the junction between the solder and
the copper substrate. 2. After 32 days of annealing, the
thicknesses of the intermetallic layers for the lead-free joints
are slightly smaller than those for 63Sn-37Pb. 3. Four-point bend
testing experiments showed that the lead-free joints are
mechanically as sound as the joints made with 63Sn-37Pb. These
combined results indicate that the higher assembly process
temperature, the nature of the lead-free ball metallurgies and the
lead-free solder paste are not a concern regarding the quantity and
the brittleness of the intermetallics for the joint systems
studied. Therefore, the reliability of these lead-free systems can
be pursued with more extensive tests, such as accelerated thermal
cycling. Acknowledgments The authors would like to acknowledge
Jerry Ortkiese of HP and Fay Hua formerly with HP, now with Intel
Corporation, for their continuous support and suggestions.
REFERENCES [1]
http://www.jeida.or.jp/english/information/pbfree/roadmap.html [2]
S. Prasad, F. Carson, J.S. Lee, T.S. Jeong, Y.S. Kim, Reliability
of Lead Free BGA Packages, Proceedings of IMAPS, Boston September
2000 [3] S. Prasad, F.Carson, G.S. Kim, J.S. Lee, P. Roubaud, G.
Henshall, S. Kundar, A. Garcia, R. Herber, R. Bulwith., Board Level
Reliability of Lead-Free Packages, Proceedings of SMTA, Chicago
September 2000, pp 272-276
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[4] Wassink, R. J. Klein, Soldering in Electronics, 2nd ed.,
1989, Electrochemical Publications Limited, Ayr, Scotland. [5] P.T.
Vianco, K.L. Erickson, P.L. Hopkins, Solid State Intermetallic
Compound Growth Between Copper and High Temperature, Tin-Rich
Solders, Journal of Electronic Materials, Vol.23, No 8, 1994, p.
721